1) Someone who voluntarily decided to stay where they are (i.e. for life).
2) (Mil.) An enlisted soldier or officer that decides to re-enlist to go career

1) This place has good pay, great benefits, and nice people. They might just make a lifer out of me.
2) "Who's that?"
"That's Captain Anderson; he started out in the 22nd Armored, but ended up here as a tech. specialist. He's thinking about becoming a lifer."

Someone in the military, especially Navy or Marines, who stays in because he's too scared of civilian life. Not all career military are lifers. Someone who is "career" is respected because they stay in because they like it, whereas a lifer hates being in the military but is too scared they will end up sleeping in their car if they get out (which sometimes happens). A lifer is basicly serving a self-imposed life sentence in the military. Lifers will kiss every ass above them in the chain of command and will try to make everyone else around them as miserable as they are.

"Shipmate, are you going to reenlist? We'll make you a yellow shirt."
"No Chief. I ain't a lifer. I'm gettin' a real job."

A person whose sole purpose in life is to succeed at their minimum wage job. They often will be "two-faced" in that they will be nice to your face and then backstab you to others in order to advance their friendly relationship with their boss. Lifers will often get upset over trivial things that are work related and rarely have good things to say about employees who do not plan on becoming lifers. Lifers will commonly be attracted romantically to lifers of the opposite sex at the workplace. They will spend an unbelievable amount of time outside of work with this person under the guise that they are just friends. When faced with the opportunity to do something that will make their life better (i.e. pursue higher education)or pick up extra hours at their work, lifers will consistently do the latter. Lifers also have a unrealistic belief that their job is better than it really is. Because of this lifers will typically talk about their job even when no one is interested. Their is no proven scientific research regarding why lifers feel thier job is so important, but scientist believe it is because lifers are typically substance abusers that lack a sense of reason.

"Scott used to be really cool. He then got that minimum wage job and now he spends all of his time with that lifer girlfriend of his discussing how to get the latest promotion to the cooler."